Non-contact shape measurement of objects is of great interest in many areas of technology, medicine, and art. In industrial applications, accurately examining the shape of machine parts and tolerances is of great importance. Recently, dentists have used non-contact shape measurement devices to image and document the three-dimensional (3D) shape of the teeth. Documentation of antique artifacts is another application for 3D shape measurement systems.
Many devices and methods have been developed for precision of 3D measurements. Among these, methods based on structured light projectors have attracted the most attention. Structured light techniques are considered to be one of the most effective, reliable, and robust optical non-contact methods for 3D surface height measurement. A common structured light projector is a fringe projector that is composed of lines with different intensities. The fringes can be generated by digital light processing (DLP), by a slide inserted in a light projector, or by two coherent light spots located a short distance apart (such as in Young's double slit experiment). Fringe projection based on Young's double slit experiment has the advantage of having infinite depth of focus with infinite spatial resolution, making it attractive for 3D imaging systems.
Several devices, methods and algorithms have been developed as the bases of 3D shape measurements systems using fringes. For example, fringe phases are shifted and imaged by an imaging sensor, such as a CCD or CMOS camera, and the phase on the surface is measured. This method is known as phase shifting interferometry (PSI).
A 2π ambiguity exists in phase determination using PSI methods and additional efforts are needed to resolve this ambiguity. Methods for resolving phase ambiguity resulting from PSI are known as phase unwrapping methods. Over the years several methods and algorithms have been developed to solve phase unwrapping problems. For instance, the use of more than one fringe projector, each with a different fringe frequency, can unambiguously resolve the phase wrapping problem.